The present invention refers to a column decoder circuit for page reading of a semiconductor memory.
It is known that reducing the time for reading semiconductor memories currently represents a problem of considerable importance.
In particular, it is known that the data contained in a memory are not immediately available as soon as a reading operation is requested. In fact, it is necessary to wait for a latency time required for decoding the addresses of the memory words to be read (addressing of the memory), for selecting the memory cells, and transferring the information on a data bus.
Column decoder circuits have thus been studied that enable simultaneous reading of a number of memory words, if these are set in consecutive memory locations (the reading of series of consecutive memory words, known as xe2x80x9cburst-modexe2x80x9d reading, is in fact a frequent case). The memory words read are supplied in succession on a data bus at every clock cycle, so as to eliminate or reduce the latency times.
For greater clarity, reference is made to FIG. 1, which shows a column decoder circuit 1 of a known type, in particular having 128 columns per bit and allowing simultaneous reading of four memory words having consecutive addresses (memory pages). The column decoder circuit 1, belonging to a semiconductor memory, comprises a first level decoder stage 2, a second level decoder stage 3, and a plurality of bit selection stages 4. In particular, the bit selection stages 4 are equal in number to the bits forming a memory word (for example, 16).
The first level decoder stage 2 has an input connected to an address bus 5, from which it receives two first level address bits, and is connected at the output to a first level bus 7 to supply four first level signals YO0, . . . , YO3, each having a selection value and a deselection value.
The second level decoder stage 3 has an input connected to an address bus 5, from which it receives three second level address bits, and is connected at the output to a second level bus 8 to supply eight second level signals YN0, . . . , YN7, each having a selection value and a deselection value.
In particular, during reading, a single first level signal YO0, . . . , YO3 and a single second level signal YN0, . . . , YN7 at a time are set at the selection value, while all the other first level and second level signals have the deselection value.
Each of the bit selection stages 4 has first level inputs, connected to the first level bus 7, second level inputs, connected to the second level bus 8, and third level inputs, connected to the address bus 5, from which they receive two third level address bits. In addition, each of the bit selection stages 4 is connected to a respective group of bit lines 10 of a memory array (not shown), and has an output connected to a data bus 11. Each group of bit lines 10 comprises, for example, 128 bit lines 10, identified in FIG. 1 by a progressive number between 0 and 127, corresponding to a column address of a memory word.
FIG. 2 shows in greater detail a block diagram of a bit selection stage 4, which comprises first level selector stages 15, second level selector stages 16, column amplifiers 14, also referred to as sense amplifiers, and a multiplexer 12.
Each first level selector stage 15 is connected to the first level bus 7 supplying the first level signals YO0, . . . , YO3, and to four bit lines 10. In particular, the bit lines 10 connected to a same first level selector stage 15 have column addresses k, k+32, k+64,k+96(k=0, 1, . . . , 31).
Each second level selector stage 16 is connected to the second level bus 8, supplying the second level signals YN0 . . . , YN7, and to eight first level selector stages 15.
The multiplexer 12 is connected to the second level selector stages 16 via the column amplifiers 14 and pass transistors 18. In addition, the multiplexer 12 is connected to the address bus 5, supplying the third level address bits, and has an output connected to the data bus 11.
FIG. 3 shows a circuit diagram of the bit selection stage 4 shown in FIG. 2.
According to the values of the first level signals YO0 . . . , YO3, each first level selector stage 15 selects and connects one of the bit lines 10 to the respective second level selector stage 16. For example, all the lines having column address k+32 (wherein k=0, 1, . . . , 31) are selected. Likewise, each second level selector stage 16, according to the values of the second level signals YN0 . . . , YN7, selects and connects a first level selector stage 15 to the respective column amplifier 14. In practice, four bit lines 10 having consecutive addresses are selected simultaneously, so that the content of four memory cells (not shown) connected to the selected bit lines 10 is available at the inputs of the multiplexer 12. If it is necessary to read one or more memory pages, in successive clock cycles, the multiplexer 12 sets the data present on its own inputs in sequence on the data bus 11.
In known column decoder circuits, however, it is disadvantageous that simultaneous reading of four memory words is possible only if the address of the first memory word to be read can be divided by four. Otherwise, it would be necessary to have available, for each second level selector stage 16, a set of independent values of the second level signals YN0 . . . , YN7, while there are present a single second level decoder stage 3 and a single second level bus 8. Suppose, for example, that it is required to read memory words having column addresses 1, 2, 3 and 4. The bit lines 10 for the column addresses 1, 2 and 3 can be selected by setting the second level signal YN0 at the selection value, which, however, selects the bit line 10 for the column address 0, instead of the one having column address 4 (FIGS. 2 and 3). Consequently, in this case it is possible to read only three valid memory words simultaneously.
According to principles of the present invention, a column decoder circuit is provided that enables reading of memory pages independently of the initial column address. A column decoder circuit is provided for page reading of a semiconductor memory that includes a circuit.